Thin film transistors (hereinafter also referred to as “TFTs”) are already widely used in a technical field of liquid crystal displays. A TFT is a kind of field-effect transistor, and is named due to the fact that a semiconductor film for forming a channel is formed with a small thickness. At present, a technique to manufacture a TFT using amorphous silicon or polycrystalline silicon as the thin semiconductor film has already been put into practical use.
A semiconductor material called “microcrystalline silicon” has been known for a long time together with amorphous silicon and polycrystalline silicon, and there also has been a report on microcrystalline silicon related to a field-effect transistor (for example, see Patent Document 1: U.S. Pat. No. 5,591,987). However, attention has not been paid on a TFT using microcrystalline silicon compared with an amorphous silicon transistor and a polycrystalline silicon transistor so far; thus, there has been a delay in practical use and reports thereof are made merely from an academic viewpoint (for example, see Non-Patent Document 1: Toshiaki Arai et al., “SID '07 DIGEST” 2007, pp. 1370-1373).
A microcrystalline silicon film can be formed over a substrate having an insulating surface, such as glass, by decomposing a source gas with plasma (weakly-ionized plasma) by a plasma CVD method; however, it has been considered to be difficult to control generation of crystal nuclei and crystal growth because reaction proceeds in a non-equilibrium state.
Various researches have been made on microcrystalline silicon. According to a hypothesis, growth mechanism of microcrystalline silicon is as follows: first, a portion of an amorphous phase, in which atoms are configured randomly, grows over a substrate, and then nuclei of crystals start to grow (see Non-Patent Document 2: Hiroyuki Fujiwara et al., “Japanese Journal of Applied Physics (Jpn. J. Appl. Phys.)” vol. 41, 2002, pp. 2821-2828). In Non-Patent Document 2, it is considered that the density of microcrystalline silicon nuclei can be controlled with the concentration of a hydrogen gas used in forming a microcrystalline silicon film because peculiar silicon-hydrogen bonds are observed on an amorphous surface when nuclei of microcrystalline silicon start to grow.
Further, influence on a growing surface of a microcrystalline silicon film due to an impurity element such as oxygen or nitrogen has also been investigated, and it has been reported that by reducing the concentration of the impurity element, the size of a crystal particle of a microcrystalline silicon film becomes large, and thus the defect density (especially, the defective charge density) is reduced (see Non-Patent Document 3: Toshihiro Kamei et al., “Japanese Journal of Applied Physics (Jpn. J. Appl. Phys.)” vol. 37, 1998, pp. L265-L268).
Further, there is a report that in order to improve operation characteristics of a TFT, the purity of a microcrystalline silicon film needs to be improved, and an attempt has been made to improve effective mobility by controlling the concentrations of oxygen, nitrogen, and carbon to be 5×1016 cm−3, 2×1018 cm−3, 1×1018 cm−3, respectively (see Non-Patent Document 4: C.-H. Lee, et al., “International Electron Devices Meeting Technical Digest (Int. Electron Devices Meeting Tech. Digest), 2006, pp 295-298). In addition, fabrication of a microcrystalline semiconductor film with improved effective mobility was reported, in which a deposition temperature in a plasma CVD was set to be 150° C. and the concentration of oxygen was reduced to be 1×1016 cm−3 (see Non-Patent Document 5: Czang-Ho Lee et al., “Applied Physics Letters (Appl. Phys. Lett.), Vol. 89, 2006, p 252101).